IRS 2092 Class D Amp Kit - Oscillation or Normal?

MEF Members -

To learn a bit more about Class D amplification, I bought a kit from ebay - LJM L15D KIT IRS2092 Power amplifier kit (with IRFB4019 MosFets). For a tad over $20, it was an interesting build. But without a schematic, it kinda defeated the point. Eventually, I did find a schematic for this board - directly from International Rectifier. The schematic is a match. The only thing that is tricky is understanding the Overload Protect and circuitry around the relay. I do not see the relay per se - I do see the "open" note next to R29A and C9A. I also see a couple Diodes on the schematic D5A and D6A near the output. I believe those are created by combining two 2N5551 transistors connected to one another in an odd way. Anyway, I am working through all that. The amp is working - yeah!!

So, my question centers on the ripple (or oscillation) that I am seeing on the Power Supply. I am using my Peavey 400BH Module Power Supply Project for the plus and minus 50 volts. The ripple (unloaded) looks like one would expect - the typical sawtooth shape. But when I connect the power to the amp board, I am seeing something totally different. It is hard to measure the amplitude and try to discern the frequency. Is this normal? I am wondering if the 2092 chip is causing this.

The amp sounds clean - I am just curious about the waveform on B= and B-.

It is a digital amp, it works at very high frequency, so I don't doubt it leaves high freq artifacts on your power rails.

The only thing that is tricky is understanding the Overload Protect and circuitry around the relay. I do not see the relay per se - I do see the "open" note next to R29A and C9A. I also see a couple Diodes on the schematic D5A and D6A near the output.

What overload protection around what relay? I see none of that.

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Education is what you're left with after you have forgotten what you have learned.

Thanks for the reply. I was thinking the ripple wave may have something to do with the IC high frequency.

As for the overload protection, this is where the schematic is not complete. In the photo below, you see the blue relay. One end goes to L1A (22uh inductor) and the other end goes directly to the output.

Just below the relay, there are two 2N5551 NPN transistors mounted where the round portion faces the other. I need to trace out the connections very carefully, but it appears they are connected together at the base. On one end you have a collector of one transistor tied to an emitter of the other transistor. That connection goes to a 100K resistor and then to a 1N4004 diode. The diode then goes to B-. The other two transistors have similar connections but they appear to go to B+. That pair of @n5551 are located to the right of the L1A inductor and below C12A, the ,47uf cap.

I will order another kit (to have a pair of amps in a small box) and when that board arrives, I will take my time to trace out these four transistors and see how they are connected into the circuit. It is alot easier to do as you install the parts, checking components and connection points (at least for the components after L1A) and not having to worry about measurements of other components that are installed.

Unless I find another schematic that is more accurate, I will revisit this post at a later date. It takes awhile to get shipments from the far east!!

"Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

Power Supply with amp connected, 55mv rms. But you can see the readout showing the frequency @ 1.258 mhz.

That's better! Thx.

Is the 55mVpp ripple at full power or zero power? If at idle I wonder what it is a full power. The thing that is crucially different about class D amps is that the power supply ripple directly passes to the the output, mitigated only by the negative feedback. In a class AB amp the power supply ripple (usually) has a much smaller effect. Therefore you do need to have a decent low ripple power supply.

The HF hash you see is just the amps switching noise. It's probably of no consequence but might be an indication that the board's power supply impedance is higher than it could be due to poor power rail design.

I happened to look at equipment (Harbinger powered monitors mostly) that use the IRS2092. That one too uses a similar power supply as to what I have. So just for the hell-of-it, I might look into a different PS design. If nothing else, good to learn.

Figure 25 of the doc you posted is similar to the schematic I previously posted, but there are some differences. The photo below comes close to the PC board but there are components that are missing. In time, I will add those components, document any differances, and repost a schematic.

So I finally received my second LJM L15D KIT IRS2092 Power Amplifier Kit and had the time to slowly trace out the circuit. The totally funky part is the output (after the 22uH inductor). Yes, there is a Songle SRD-48VDC-SL-A relay protecting the output that is triggered by some type of protection circuit. I numbered the four 2N5551 transistors so that I could trace out the connections. For the newbies like me, the internal relay switch closes when power is applied. Otherwise, it looks like an open circuit in the datasheet. A (moderate) Starbucks Coffee card will be sent to the first person that can figure this out the protection circuit and explain it laymanís terms to the rest of us!!

I needed a big sheet of paper to draw this out and since I could not scan and 11x17, I will give you the circuit in three pieces. You can compare what I have vs the circuit in post #10. That schematic comes from International Rectifier and is a typical application circuit.

Youíll notice components that are different, components that are on the board but not on the schematic (which is a typical hookup for the IRS 2092 chip), and some components on the schematic that are not on the board.

Why do this? The kit that I bought from eBay did not come with a schematic. And when I contacted the company that sold the kit, they did not have a schematic. Go figure. For those who want to build the kit but have the schematic too, well now you have it. And it is a good way to get an introduction to Class D amps.
Of course, I could have made an error in my circuit drawing. Back in the old days working at National Semiconductor, we had Beer Bust sessions. This is where the Techs and Engineers would review circuits and the Designer would buy you a beer if you found an error. With todayís Social Distancing, I cannot offer that. But please feel free to challenge my drawing and I will take another look. But once the board is assembled, it gets tricky to trace out.

I'd suggest you follow through the ends of the relay coil again (output.jpg). Current must flow through the coil. Transistor bases can not flow any appreciable current.

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"Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

Crapola... I forgot to draw a point to B+ where the right side of the relay meets the 390K. So we have 50v on one side of the relay coil and 2v on the other side that goes to the bases of T1 and T2.

(Blame it on the double sided PC board !!!)

Edit: There is .52v on the bases of T3 and T4. The Emitter of T4 is at 0v. There is 1.96v at the bases of T1 and T2. The Emitter of T1 is at .4 volts and the Emitter of T2 is 0v. So it looks like those two transistors are not turned on.

The coil still needs a current path to energize. The bases of T1 and T2 can not provide that. Is it possible the pinout is mixed up?

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"Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

Again, the amp is working. But the voltages in this section are bizaro!

There is 48 volts across the relay so the output is connected 22uH inductor! My iTunes sounds fine!!

One thing about the transistor pairs, they are installed back-to-back. You can see from the images below, that the bases of T1 (right side) and T2 (left side) are connected (center traces). You can also see that the Emitter of T1 is connected to the Collector of T2 (bottom view image, bottom traces). There is nothing else connected there.

There is 2.0v on the bases of T1 and T2. The Emitter of T1 has .4v. So that transistor is not turned on, or is it? The Emitter of T2 is at 0V. In fact, that point is tied to Ground. I updated the schematic.

There is .6v on the bases of T3 and T4. The Emitter of T4 is at 0v. However, the Collector is at 0v (because the connected points go to Ground). The same scenario with T3. We have the proper Base to Emitter voltage but the Collector is at 0v - odd indeed.

I tried to be very careful tracing this out while I inserted the parts. I checked the continuity from point to point during assembly.

That cannot be right. The relay probably has a diode across it coil. The resistor cannot be 390k, more like 390R if it is in series with the relay. The relay must connect to a transistor collector (most likely) or emitter. There will also be a connection from the main output to the transistors so they can detect any DC offset. Back to bank transistors are often used to detect bi-polar voltages, one doe the positive and the other the negative.

I'd expect something like main out->resistor->back to back transistors-> relay driver transistors -> relay with resistor and back emf diode.

I have been looking for equivalent circuits or to see if anyone has posted this somewhere. That resistor is indeed 390K. I posted a larger image of the PC board. You can see that resistor on the left side of the PC board, near the Speaker Ground.

Before installing any components, especially the resistors, I measured them with my DMM. The color bands are hard to make out and who knows, someone might have made an error in grabbing a resistor from a parts bin.

As we know, this circuitry is NOT needed for the amp to work. But my curiosity got the better of me, so I decided to install it and see what would happen.

One thing we do know.... if the relay no longer had 48 volts across it, the contacts would open. So for the left side of the relay's coil... if that were to go "high", you would not have the 48v across it. So the contacts would open. And that is the question. What would make this circuit change it's state.

Can you provide a clear image of both sides of the unpopulated board and one of the populated board? Maybe the relay is not a mechanical type. Still if the supply is 48V and the series resistance is 390K that's only 125uA and not enough even for a solid state relay.

According to your diagram, the relay contacts are in series with the speaker and therefore the relay has to be populated and the contacts closed for the amp to work. This is in direct contradiction of the statement "this circuitry is not needed for the amp to work".

Still if the supply is 48V and the series resistance is 390K that's only 125uA and not enough even for a solid state relay.

Earlier he had mentioned there was a B+ connection to that side of coil, so 390K not in series.
Updated version of 'output.jpg':

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"Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

My comment of "this circuitry is not needed for the amp to work" means that the PC board would work if I was to remove the protection circuitry, remove the relay, and hard wire the to the 22uH inductor. After the inductor, the only thing that is needed would be C12, the .47uf cap (250v) to ground, the 10 ohm in series with C14 (the .1uf) to ground, and the 2.2K resistor to ground. The typical application schematic (generic schematic) has those items. (See schematic in #10). I have seen other applications where the only thing after the 22uH inductor is the .47uf cap.

Pic 3 is the front end to this. With the exception of the components that I noted in the post above, there is nothing different compared to the typical application hookup.

Pic 2 is from the heat sink to the back end. You can see the blue relay, the 22uH inductor next to that (black component), and the big grey component is the .47uf cap I just mentioned.

Pic 1 has the .47uf cap and just in front of that is the 2.2K that goes to ground (mentioned above). Next to that is one of the 4004 diodes and next to that is 100K that connects between the 4004 diodes. In this picture, you can also see T3 (next to the electrolytic caps) and T4 which is next to the 2.2K. The larger of the electrolytic caps (Blue) is the 200uf cap. The smaller cap next to that is the 10uf.

Pic 1 also shows T1 and T2 tucked in the corner next to hte relay. T2 is next to the relay. T1 is near the pc board mounting hole.

I see that two IN4004's and the 100K go to the amp output, not 0V as drawn.
D33 is in parallel with the relay coil which needs about 7.5mA to operate. That's quite small.
I think the transistor pinout is ECB not EBC which I think will make them into darlington pairs.

The Collector is in the middle !! I need to find another version of the spec sheet and save it for reference. When I used my meter previously, I just checked to see if it showed a working NPN. I did not look carefully at the pin out mapping.

Nick, you da man. That changes everything. I will go back and redraw that schematic and look at the other items you mention above.

Nick was right, these transistors are hooked up as some type of Darlington pair. I am still suspicious when it comes to the base of T4 and Emitter of T3 both going to Ground. But I am looking at the traces and measuring the voltage and there it is.

With respect to the 100K that connects to the 4004 diodes. My previous "0" meant I was measuring 0 volts. I did not mean to imply that it was connected to Ground. But I do not see that point connected to the Output.

I don't know if there is connectivity happening between the two sides of the board. All I can do is draw what I see and take voltage measurements.

Yes, that is true. The big holes are easy to trace. It is the smaller holes that are the challenge - like the one next to D34. It is the hidden traces that are challenging.

The BIG diode (D3) hanging off pin 15, with the help of the mother of all magnifying glasses, that one is marked as ON 4UD R532. I looked that one up. It is called a Damper Diode, rated at Reverse Voltage of 1500v and Forward Current of 3 amps.

One thing that I need to do - there are some zener diodes on the board that I have identified - ie Z1, Z102, Z103, Z104. You can see these on the bottom side of the PC board. I also assume the other diodes like D32, D33, D34, those must be something equivalent to 1N4007s.

My friend is ordering one of these boards. We will continue to trace this out with no extra components. So I am hoping the next version of the schematic will include the zeners I mentioned above.

And at some point in time, I will send a link to this thread to the manufacturer of the board. I know he is on the DIY Audio site. But in reading some of his replies to questions, they are very cryptic.

With respect to the 100K that connects to the 4004 diodes. My previous "0" meant I was measuring 0 volts. I did not mean to imply that it was connected to Ground. But I do not see that point connected to the Output.

You can follow it right to one of the relay 'switch' pins. So it has to be the output.

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"Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

I used a different numbering. The 100K and 20uf are a low pass filter to remove audio so only allow near DC to appear on Q1 and Q2. If the voltage there is > 0.7V Q1 will turn on, if < -0.7V Q2 will be on. Thus if the DC voltage on the output exceeds +/- 0.7V the collector of Q1/Q2 will go low. That will turn off Q3/4 and the relay will open so protecting the speaker. I'm ignoring D32 in your diagram.

Here it is.... a final stab at the full schematic for the LJM L15D Version 2 Amplifier. I was able to scrunch in Nick's output protection diagram in the upper right. I crossed out resistor and cap values that were different from the base schematic. I added a few notes are well.

The eBay seller is now shipping a version 3 of this kit. There are few changes from version 2. So if you are thinking about building a stereo system, buy two boards.

Again, thanks to G1 and Nick B for contributing to the thread.

Update 4/10/2020: I forgot to mention that for Version 3, the two Mosfets have been replaced by a single IRFI4019H-117P, 5 pin Mosfet.

A while back you asked what schematic capture I used and I forgot to respond. It was ltspice. Whilst it's really a simulator you enter the data using a schematic. It's particularly fast and easy to use as a sketch type tool. What it is not a a real schematic capture package i.e doesn't have the hooks needed for manufacture.